1. Field of the Invention
The present invention relates to a new process for the manufacture of aliphatic and/or cycloaliphatic and/or araliphatic allophanates having isocyanate groups.
2. Description of the Prior Art
Polyisocyanates containing allophanate groups are known. According to GB-A 994,890, they may be obtained by reacting isocyanates having urethane groups with excess quantities of diisocyanates, either thermally or in the presence of known catalysts such as metal chelates, metal carboxylates or tertiary amines.
A disadvantage of the (cyclo)aliphatic polyisocyanates, in particular, is that the thermal process requires very long reaction time at approx. 130-135.degree. C., which causes yellowing, or the catalytic process results in the formation of considerable impurities, such as unreacted urethanes, isocyanurates and uretdiones having isocyanate groups.
A process is also known for the manufacture of (cyclo)aliphatic allophanates having isocyanate groups (e.g., DE-A 2,729,990) by reacting excess quantities of (cyclo)aliphatic polyisocyanates with (cyclo)aliphatic isocyanates having urethane groups in the presence of strong acids which form a mixed carbamic anhydride with (cyclo)aliphatic isocyanates.
A disadvantage of this process is the content of residual monomer in the product and the acid catalyst. The catalyst has to be removed with propylene oxide, for example, which requires a time-consuming additional reaction step.
From DE-A 2,165,023 it is known that when reacting oxadiazinetriones having anate groups (formula (II), page 3) with active hydrogen compounds such as alcohols, the two terminal isocyanate groups are reacted to form an oxadiazinetrione having urethane groups in a first reaction step and then in a second, very slow step the oxadiazinetrione ring is opened to the allophanate. This second reaction step is accelerated by means of very specific tetraalkyldistannoxane catalysts.
It is an object of the present invention to overcome the disadvantages of the prior art processes.
This object may be achieved in accordance with the process of the present invention. It has surprisingly been found that it is possible to carry out the second reaction step first and almost completely dispense with the first reaction step, if, according to the invention, oxadiazinetriones having isocyanate groups are reacted in the presence of basic catalysts with primary or secondary alcohols.